The present invention relates generally to high strength nonmetallic tubular structures and, more particularly, to a composite tubular structure exposed to extreme internal pressures and temperatures and the method of making the same.
While not limited thereto, the present invention is particularly adapted to form an improved composite tubular structure for use as a gun barrel resistant to abrasion and corrosion. Gun barrels have traditionally been formed of steel and steel alloys and have admirably served the purposes for which they were intended. However, advancements in ammunition technology providing improved accuracy with high muzzle velocities has also imposed limitations on existing steel gun barrels. The use of such improved ammunition generates higher propellant gas temperatures and rates of fire with resultant increases in gun barrel temperatures beyond the working range of steel. Wear and erosion rates are increased at these higher temperatures to severely limit the life cycle of steel gun barrels. For example, the M61 automatic weapon has only a 30,000 round life at a 1,000 round per minute firing rate for each barrel.
As a result, many attempts have been made to develop nonmetallic gun barrels. One known expedient is to form the gun barrel liner, which is directly exposed to the high temperature propellant gases, of a ceramic material. While monolithic ceramics have inherently low tensile strength and exhibit brittle failure characteristics, recent improvements in ceramic technology have significantly improved the strength and fracture toughness of those materials. For example, the addition of whisker reinforcement of silicon carbide (SiC) has dramatically increased the toughness and strength of ceramics. Other approaches to ceramic composites have been developed on the molecular scale by transforming the phase of included particles to toughen the ceramic. Moreover, fabrication techniques are being improved to increase the reliability and consistency of ceramic properties. Nevertheless, gun barrel liner stresses during rapid firing will exceed even the improved tensile strengths of ceramics. In order to employ ceramics as gun barrel liners, they must be precompressed. It has been demonstrated that those ceramic barrel liners which can be successfully precompressed show promising results. The classical method of precompressing a ceramic liner consists of shrink fitting a steel sleeve over the liner. While this shrink fitting technique has merit and has been partially successful, it does pose certain problems. For example, tolerance on diameters, variable coolin of the steel, and nonuniform contact stresses have resulted in cracking of the ceramic liners during the precompression process. In use of the finished product, a major limitation is expansion of the steel sleeve at a greater rate than the ceramic liner which tends to offload the precompression initially induc therein. This requires over compressing the ceramic liner fabrication and the increased shrink fit necessary creates significant manufacturing difficulties. Also, the higher temperatures required for over compressing produces axial thermal expansion of the ceramic liner resulting in undesirable tensioning thereof.